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Contract No. DE-AC36-08GO28308

Energy Impacts of Oversized 

Residential Air Conditioners—

Simulation Study of Retrofit 

Sequence Impacts 

C. Booten, C. Christensen, and J. Winkler

National Renewable Energy Laboratory

Technical Report

NREL/TP-5500-60801

November 2014
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Energy Impacts of Oversized 

Residential Air Conditioners—

Simulation Study of Retrofit 

Sequence Impacts

C. Booten, C. Christensen, and J. Winkler

National Renewable Energy Laboratory

Prepared under Task No. BE4R.0102

Technical Report

NREL/TP-5500-60801

November 2014
NOTICE

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iii

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Abstract

In this simulation-based study, magnitudes and causes of energy penalties are investigated for 

oversized residential air conditioners. Oversizing can result from overly conservative sizing of a 

new air conditioner or out-of-sequence retrofits (installing a new air conditioner first and then 

later making building improvements that reduce the cooling load). Conventional wisdom states 

that older, oversized air conditioners have significant energy penalties caused by increased onoff cycling relative to a correctly sized unit. Comparison of simulation results for different 

scenarios demonstrated that the energy penalty can be significant under certain circumstances. 

However, this is not primarily the result of increased on-off cycling. Oversizing does cause some 

additional cycling, but this does not lead to significantly increased energy use because all units 

already cycle frequently during most cooling season hours when conditions are less extreme than 

design conditions. However, if off-cycle parasitic power is present and is proportional to cooling 

capacity, the energy penalty can be significant. Also, oversizing penalties either increase or 

decrease with duct losses, depending on whether the air conditioner includes off-cycle parasitic 

power consumption proportional to air conditioner capacity.
iv

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Table of Contents

Introduction .................................................................................................................................................. 1

Literature Review.......................................................................................................................................... 1

Modeling ....................................................................................................................................................... 2

Air Conditioner Cycling ............................................................................................................................. 2

Parasitic Power ......................................................................................................................................... 4

Ducts ......................................................................................................................................................... 6

Simulations.................................................................................................................................................... 7

Scenarios................................................................................................................................................... 7

Air Conditioner Sizing................................................................................................................................ 8

Results........................................................................................................................................................... 8

Annual Energy Simulation Results ............................................................................................................ 8

Cycling Losses in Right-Sized Air Conditioners.......................................................................................... 9

Oversizing Penalties Without Off-Cycle Parasitic Power Consumption.................................................... 9

Oversizing Penalties With Off-Cycle Parasitic Power Consumption ....................................................... 10

Changes in Oversizing Penalties With Ducts Present ............................................................................. 10

Conclusions................................................................................................................................................. 11

References .................................................................................................................................................. 13

Appendix: Effects of Part Load Ratio .......................................................................................................... 14
v

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List of Figures

Figure 1. Linear model for PLF as a function of PLR. This assumes that there are no parasitic 

power draws or that they are accounted for separately................................................................... 4

Figure 2. PLF’ including parasitic power draws. The linear PLF model is included for reference..... 6

Figure 3. Histogram of loads for a right-sized unit and a 1.8x oversized unit. Dotted lines represent 

the PLR where the PLF is being calculated..................................................................................... 14

Figure 4. PLF for two different cycling degradation coefficients. The vertical lines are translated 

over every PLR.................................................................................................................................... 15

Figure 5. Comparison of PLF curves with (nonlinear) and without (linear) parasitic losses............ 16

List of Tables

Table 1. House Characteristics.................................................................................................................. 7

Table 2. Simulated Air Conditioner Cooling Capacity............................................................................. 8

Table 3. Total Cooling Electricity (kWh/yr)............................................................................................... 8

Table 4. Total Cooling Electricity (kWh/yr) Without Cycling Losses..................................................... 9

Table 5. Cycling Losses (%) With Right-Sized Air Conditioners............................................................ 9

Table 6. Oversizing Penalties (%) Without Off-Cycle Parasitic Power Consumption.......................... 9

Table 7. Oversizing Penalties (%) With Off-Cycle Parasitic Power Consumption.............................. 10
1

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Introduction

Residential retrofits can be substantial investments that affect home energy use for years. 

Because many retrofits are expensive, homeowners often stage them over months or possibly 

years. For some retrofits, homeowners don’t take action until they face equipment failure. Then 

they replace a piece of equipment such as an air conditioner immediately and often have little 

time to plan for the best return on investment or highest energy savings. Air conditioner 

oversizing results from overly conservative sizing of a new air conditioner or out-of-sequence 

retrofits (installing a new air conditioner first and then later making building improvements that 

reduce the cooling load). Homeowners need to have access to the most accurate information 

available so they understand the energy impacts of decisions made in these situations.

Conventional wisdom suggests that oversized cooling equipment leads to higher energy use 

because equipment cycling is inefficient (or at least that this was the case with older air 

conditioners). This reasoning provides impetus for performing intended retrofits before—or

simultaneously with—an air conditioner replacement so the new air conditioner can be sized 

appropriately for the retrofitted load. Otherwise, if an air conditioner is replaced with an 

identically sized unit, it may not operate as efficiently after the additional retrofits, because the 

oversizing will lead to additional cycling losses. On the other hand, total duct energy losses are 

roughly proportional to air conditioner runtime, and a correctly sized system with longer runtime 

compared to an oversized system would experience more duct losses, potentially offsetting the 

expected energy penalty from oversizing. Quantitative analysis is necessary to determine if 

conventional wisdom is accurate and, if so, what are the relative magnitudes of the various

effects. 

This report describes a simulation-based case study of a typical 1960s vintage home undergoing

retrofits including air conditioner replacement, duct improvements, and envelope improvements. 

The duct and envelope upgrades reduce the cooling load on the house and provide an 

opportunity for reducing the size of the air conditioner. Simulations were performed with air 

conditioners of both the original sizing (oversized) and appropriately downsized (right-sized) for 

the upgraded house. Specific questions that are addressed are:

• How much of an air conditioner’s energy use is a result of cycling?

• What is the increase in energy use caused by additional cycling from an oversized air 

conditioner?

• Under what circumstances does oversizing lead to significantly increased energy use? 

• How are oversizing energy penalties influenced by the presence of ducts?

This study does not address sizing impacts on air conditioner cost or the dehumidification effects 

associated with cycling of equipment.

Literature Review

Many researchers have studied the effects of oversizing air-conditioning equipment. A U.S. 

Department of Energy factsheet presents simulations that show a 13% increase in energy use due 

to doubling the size of an air conditioner in an energy efficient house (DOE 2002). Proctor et al.

(1995) state that oversizing will increase energy use but don’t provide estimates on the level of 
2

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increase. Cummings et al. (1997) collected data on 368 homes in Florida and found 3.7% and 

9.3% increases in energy for 20% and 50% oversizing, respectively. The t-value indicated a 

difference in energy use at the 95% confidence level, but the R2 of 0.37 suggests that variation 

among samples (for example through house characteristics or occupant behavior) could play a 

significant role in the oversizing penalties observed. Sonne et al. (2006) studied three houses in 

Florida with air conditioner energy use data before and after replacing units with different 

capacity units that were otherwise identical. This study found mixed results in terms of having 

any energy penalty. Gorter (2012) states that oversizing leads to higher energy bills but doesn’t 

provide quantitative information. Rhodes et al. (2011) studied installation and design impacts of 

air conditioner systems on energy use in Austin but did not find that oversizing leads to higher 

energy for individual houses. An Energy Center of Wisconsin report (2008) showed inconclusive 

results about the energy impacts of oversizing in two homes, each of which had otherwise 

identical 2- and 3-ton systems installed. Henderson et al. (2007) performed simulations in hothumid climates with multiple air conditioner capacities and found that oversizing had very little

impact on overall energy use. 

Modeling

Air Conditioner Cycling

In simulation programs such as DOE-2 and EnergyPlus (DOE 2013), air conditioner cycling is 

modeled using average performance degradation effects of cycling over a given time step rather 

than modeling individual on-off cycles. A detailed derivation of the following model, and 

references, are given in Henderson (2000).The modeling begins with a part load ratio, PLR, 

sometimes called a cooling load factor, CLF

PPP = ssssssss ccc c ll

sss s − ss ss ssssssss ccc c c c c

PLR is used to calculate the part load fraction, PLF, sometimes referred to as the part load factor 

or part load function 

PPP = a ll ee

sss s − ss ss ee

A simple and common method for relating PLF to PLR is through a concept called the 

degradation coefficient, CD. The value of CD is typically between 0.05 (lower cycling loss) and 

0.25 (higher cycling loss); the values used in this study for the particular air conditioner units 

simulated are between 0.07 and 0.2. Modern, single-stage air conditioners tend to have CD values 

around 0.07.

PPP = 1 − CD(1 − PPP)

EnergyPlus defines the runtime fraction, RTF as follows

R ≡

PPP

PPP = PPP1 − CD(1 − PPP)
3

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Thus, cyclic losses are accounted for by increasing the required air conditioner runtime to meet 

the building load. The cooling energy consumption is 

E = R ∗

C C

CC

The difference in energy consumption between two air conditioners, one right sized (RS) and 

one oversized (OS), is then

O O P = R OO �

C C

CC �

O

− R R �

C C

CC �

R

The rated cooling capacities and the PLR of the two units can be related by a capacity ratio, CR

C ≡

C C OO

C C R

= PP R

PPPOO

Substituting into the oversizing penalty equation above and assuming the COP of the air 

conditioners is identical 

O O P = PPPR � 1

PPPOO

− 1

PPPR

� �

C C R

CC �

The percent energy penalty is then 

% O O P = 100 ∗ O O P

ER

= 100 ∗

PPPR � 1

PPPOO − 1

PPPR � �

C C R

CC �

R R ∗ �

C C R

CC �

= 100 ∗ PPPR � 1

PPPOO

− 1

PPPR

�

= 100 ∗ �

PPPR

PPPOO

− 1�

Figure 1 plots the PLF as a function of PLR for the bounding values of CD used in this study. A 

value of 0.2 can be interpreted as meaning that the degradation in performance with infinitesimal 

load (PLR ~0) is 20% lower than if the unit operates continuously (PLR =1); a value of 0.07 

means performance would degrade by 7% under the same infinitesimal load. In simulation, a 

PLR near zero is possible depending on the load; as a practical matter, this extreme is almost 

never encountered. Minimum hourly PLR values are typically around 0.1. The exact value is 

controlled by the thermostat, which often has a minimum on-time of approximately 5 minutes. 

This means that either the air conditioner does not turn on at all, PLR = 0, or if it turns on just 

once during an hour the PLR is 0.083 if the on-time is exactly 5 minutes. If most hours of the 

year have PLR values well away from the extremes, the degradation caused by cycling with very 
4

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poor performance (CD = 0.2) will be substantially less than 20% and with very good performance 

(CD = 0.07) will be substantially less than 7%.

Figure 1. Linear model for PLF as a function of PLR. This assumes that there are no parasitic 

power draws or that they are accounted for separately.

Henderson (1991) presents a more physical interpretation of CD by relating it to the time constant 

of the air conditioner and the per-hour cycling rate of the air conditioner. A typical time constant 

at the time that paper was published was around 80 seconds. The maximum cycling rate of the 

air conditioner depends on the physical characteristics of the building and on the thermostat. A 

cycling rate of 3.125 cycles per hour corresponds well with a CD = 0.25 with a time constant of 

76 seconds (Henderson 1999). A lower maximum cycling rate, or a smaller time constant, results 

in cycling performance that is comparable to lower CD values.

Parasitic Power

The above formula for calculating PLF assumes either there is no off-cycle parasitic power

consumption in the air conditioner or that this energy consumption is accounted for separately in 

the simulation.

A commonly used modification to the PLF correlation includes off-cycle parasitic losses directly 

(Henderson et al. 2000). This approach is introduced here because it is a well-known method 

from the literature and it clearly illustrates the effects of off-cycle parasitic power draw on PLF.

An example of a parasitic loss is a crankcase heater on the air conditioner. It is used to keep the 

oil in the compressor warm so that it does not absorb refrigerant when the air conditioner is not 

operating; it is standard equipment on a heat pump and can be used on an air conditioner as well. 

Crankcase heaters and their control strategies vary by air conditioner manufacturer, vintage, and 

price point. A crankcase heater typically operates when the air conditioner compressor is not 
5

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running and, if thermostatically controlled, when the temperature of the compressor discharge

line temperature is below a set point such as 65°F.

Parasitic power can be represented as a fraction of unit operating power

= P p

P h

This can be used to formulate a modified part load fraction that includes parasitic power effects, 

PLF’ (Henderson 1999)

PP ′ = PPP

�

PPP

PPP + �1 − PPP

PP � �

If pr is constant, parasitic power losses are proportional to operating power consumption. For a 

given system efficiency this is proportional to capacity. If pr is zero, PLF’ reduces to PLF from 

the linear correlation above. The lower PLR values that result from oversizing and the consequent lower runtime fractions mean the parasitic power is a larger wattage as well as being “on”

for more time. This combination can lead to an oversizing energy penalty even if CD is zero.

Figure 2 shows the PLF’ correlation with two different pr values, a linear correlation for 

comparison, as well as a curve that is representative of the simulations in this study that have 

parasitic losses. The pr values are the bounding values considered by Henderson et al. (1999) for 

“good” (pr 0.01) and “bad” (pr 0.03) air conditioner units. The curve that is representative of

simulations in this study is for 20 W/ton of nominal cooling capacity of parasitic power and COP 

3.07 (corresponding to a SEER 10 unit), which gives a pr of 0.02. Note that for PLR values 

above approximately 0.5, parasitic power draws of this magnitude result in minor differences

with the linear model. 
6

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Figure 2. PLF’ including parasitic power draws. The linear PLF model is included for reference. 

The power ratios (pr) of 0.01 and 0.03 are representative of “good” and “bad” air conditioner units 

from Henderson (1999). The modeling approach used in this analysis is based on specifying 

parasitic power as a function of air conditioner capacity only. The 20 W/ton of nominal cooling 

capacity of parasitic power and COP 3.07 (SEER 10) correspond to a pr of approximately 0.02. For 

the range of SEER values simulated here, the pr values are 0.017–0.027.

Ducts

Ducts have an important impact on air conditioner performance. Location (conditioned space 

versus unconditioned space such as an attic), leakage, and conduction losses all have substantial 

effects on air conditioner performance. All ducts were assumed to be in the attic for this study.

The leakage was specified as a percentage of total airflow, so there is no correction for lower 

operating pressures caused by downsizing the air conditioner and leaving the ducts at the original 

size. 

The fan efficiency is modeled as constant; it is not a function of pressure or flow rate. Higher 

efficiency air conditioner units have more efficient fans, but for any individual unit the efficiency 

is constant. The fan flow rate per unit capacity (CFM/ton) is constant over all units considered in 

this study.

Duct conduction losses are calculated using the log-mean temperature-difference model, which 

uses the air handler supply temperature, Tair handler, the air temperature in the duct location, Tattic,

(all ducts were assumed to be in the attic), and the overall heat transfer coefficient, UA

Qcccc = UU �

( aaa ha aaa − aaaa ) − � r r − aaaa �

ln � aaa ha aaa − aaaa

r r − aaaa �

�
7

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The UA is based on the duct insulation R-value, RDucts, an exterior film resistance that was

assumed to be 1.7 Btu/(h-°F-ft2 ) (ASHRAE 1989) and the exterior surface area of the ducts, A. 

The interior film coefficient is assumed infinite such that the inner wall of the ducts is at the 

same temperature as the supply air. 

UU = DD + 1.7

Simulations

Simulations were performed using BEopt 2.0 with EnergyPlus v8.0 as the simulation engine with 

TMY3 weather data for Houston, Texas.

Scenarios

Two retrofit-sequence scenarios are included in the analysis.

1. Right-sized (RS)—Air conditioner replacement is appropriately sized for the load after 

all retrofits are completed.

2. Oversized (OS)—Air conditioner replacement is the same capacity as the original unit 

and is oversized after retrofits are completed.

In both cases, post-retrofit buildings are simulated. Pre-retrofit simulations were not required 

since the comparisons are between different sized air conditioners, not between buildings with 

different characteristics.

Important house characteristics are presented in Table 1. The structure is a 1,280-ft2 1960s 

vintage house with operation based on the Building America House Simulation Protocol 

(Hendron and Engebrecht 2010).

Table 1. House Characteristics 

Characteristic Pre-Retrofit Post-Retrofit

Floor area 1,280 ft2

Foundation Slab on grade

Windows Double-pane, clear glazing, U 0.76, 

SHGC 0.67

Infiltration 15 ACH50 3 ACH50

Attic insulation R-11 blown cellulose R-30 blown cellulose

Walls Uninsulated 2×4 walls 16 in. o.c. R-13 2×4 walls 16 in. o.c.

Ducts, in attic R-4, 20% leakage R-8, 2.5% leakage

Air conditioner SEER 10

Each of the following was 

considered for all scenarios

SEER 10 

SEER 13

SEER 16, single speed

SEER 16, two speed
8

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Air Conditioner Sizing

The simulation results in Table 2 provided the air conditioner sizing for subsequent simulations; 

they are not directly related to further simulations except for sizing. The units are sized according 

to the implementation of ACCA Manual J in BEopt. Simulations for each pertinent combination 

of house characteristics were performed with and without parasitic losses to isolate the effects on 

annual energy use. Note that subsequent simulations only considered two duct options: none or 

upgraded. Later simulations that had 6.5 ton capacity systems assumed upgraded ducts.

Table 2. Simulated Air Conditioner Cooling Capacity

Ducts Envelope Capacity (tons) Capacity Ratio

None Original 3.5 1.8 None Upgraded 2

Original Original 6.5 2.2 Upgraded Upgraded 3

Results

Annual Energy Simulation Results

Table 3 shows total annual cooling energy, including fan energy, for the retrofit scenarios

simulated with upgraded ducts and without ducts, different air conditioner sizing and

characteristics, and parasitic power losses.

Table 3. Total Cooling Electricity (kWh/yr)

(blue represents lower values, red represents higher values)

Ducts

Present

Capacity

(tons)

Capacity

Ratio

Parasitic

Power 

(W)

SEER 10

CD = 0.2

SEER 13

CD = 0.07

SEER 16

CD = 0.07

SEER 16 2spd

CD = 0.11

No

2 RS

1.8

0 3638 2704 2185 2115

3.5 OS 0 3783 2737 2210 2173

No

2 RS

1.8

40 3928 3000 2481 2394

3.5 OS 70 4334 3294 2768 2714

Yes

3 RS

2.2

0 4026 2928 2367 2364

6.5 OS 0 4066 2885 2331 2335

Yes

3 RS

2.2

60 4485 3395 2835 2810

6.5 OS 130 5137 3965 3412 3397
9

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Cycling Losses in Right-Sized Air Conditioners

Table 4 shows, as a theoretical reference, results for air conditioners with no cycling losses 

(CD = 0).

Table 4. Total Cooling Electricity (kWh/yr) Without Cycling Losses

(blue represents lower values, red represents higher values)

Ducts

Present

Capacity

(tons)

Parasitic

Power

(W)

SEER 10

CD = 0.0

SEER 13

CD = 0.0

SEER 16

CD = 0.0

SEER 16 2spd

CD = 0.0

No 2 0 3157 2578 2087 2001

Yes 3 0 3387 2766 2238 2192

Table 5 shows cycling losses for right-sized air conditioners (without off-cycle parasitic power 

consumption). The values are calculated by subtracting the theoretical no-cycling values in Table 

4 from the corresponding simulation result in Table 3, using the no-cycling values as the 

reference for the percentages. Significant cycling losses occur even with right-sized air 

conditioners, because most hours with cooling have relatively low loads compared to the air 

conditioner capacity, which is determined using the design cooling load.

Table 5. Cycling Losses (%) With Right-Sized Air Conditioners

(blue represents lower values, red represents higher values)

Ducts

Present

Capacity

(tons)

Parasitic

Power

(W)

SEER 10

CD = 0.0

SEER 13

CD = 0.0

SEER 16

CD = 0.0

SEER 16 2spd

CD = 0.0

No 2 0 15% 5% 5% 6%

Yes 3 0 19% 6% 6% 8%

Oversizing Penalties Without Off-Cycle Parasitic Power Consumption 

Table 6 shows air conditioner oversizing penalties (without off-cycle parasitic power 

consumption). The values are calculated from results rows 1-2 and 5-6 in Table 6 by subtracting 

right-sized energy values from the oversized energy values for each scenario, using right-sized

values as the reference for the percentages. Without off-cycle parasitic losses, oversizing 

penalties range from –2% to 4% for capacity ratios of 1.8 and 2.2. When off-cycle parasitic 

power consumption proportional to air conditioner capacity is not present, oversizing penalties 

are not very significant (and are even negative in some cases because of reductions in duct 

losses).

Table 6. Oversizing Penalties (%) Without Off-Cycle Parasitic Power Consumption

(blue represents lower values, red represents higher values)

Ducts

Present

Capacity

Ratio

Parasitic

Power

SEER 10

CD = 0.2

SEER 13

CD = 0.07

SEER 16

CD = 0.07

SEER 16 2spd

CD = 0.11

No 1.8

None

4% 1% 1% 3%

Yes 2.2 1% –1% –2% –1%
10

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Oversizing causes additional cycling beyond what occurs in right-sized air conditioners during

part-load conditions, but there are diminishing effects on efficiency. With a CD value of 0.2, for 

example, an hour when the right-sized PLR is 0.4 has efficiency degradation of 12% (PLF of 

0.88); an oversized (by a factor of 2) unit has a PLR of 0.2 and efficiency degradation of 16%

(PLF of 0.84). As described previously the hourly oversizing penalty is calculated as the 

difference between the reciprocals of the PLF values multiplied by the original PLF,

approximately 5% in this example. An annual oversizing penalty depends on aggregation across

hourly results with many PLR values.

In cases with duct losses (the second row of Table 6) penalties are lower, because oversizing 

reduces annual duct losses (see the description in the Oversizing Penalties with Duct Losses 

section below).

Oversizing Penalties With Off-Cycle Parasitic Power Consumption

Table 7 shows oversizing energy penalties in cases with off-cycle parasitic power consumption

proportional to air conditioner capacity. The values are calculated from results rows 3-4 and 7-8

in Table 7 by subtracting right-sized energy values from the oversized energy values for each 

scenario, using right-sized values as the reference for the percentages. With off-cycle parasitic 

losses, oversizing penalties range from 10% to 21% for capacity ratios of 1.8 and 2.2.

Table 7. Oversizing Penalties (%) With Off-Cycle Parasitic Power Consumption

(blue represents lower values, red represents higher values)

Ducts

Present

Parasitic

Power

Capacity

Ratio

SEER 10

CD = 0.2

SEER 13

CD = 0.07

SEER 16

CD = 0.07

SEER 16 2spd

CD = 0.11

No

20 W/ton

1.8 10% 10% 12% 13%

Yes 2.2 15% 17% 20% 21%

The oversizing penalties are significantly larger when off-cycle parasitic power consumption is 

proportional to air conditioner capacity. Oversizing leads to higher parasitic power and more offcycle time and, therefore, increased off-cycle parasitic energy consumption. The oversizing 

energy penalty is a moderately strong function of SEER; the higher percent penalties at higher 

SEER values are due to the lower overall energy use in the more efficient systems compared to 

the parasitic energy consumption.

Changes in Oversizing Penalties With Ducts Present

In the presence of duct losses (before and after retrofit) the larger oversizing capacity ratios seen 

in Table 4 would be expected to cause more cycling and larger oversizing penalties.

However, without off-cycle parasitic power consumption, oversizing penalties are reduced in the 

presence of ducts as indicated by Table 6. Conduction heat transfer through the duct walls is 

approximately proportional to the runtime of the unit. Oversized units run for shorter times and 

hence have lower losses relative to right-sized units. 

Off-cycle parasitic power consumption proportional to air conditioner capacity presents a

tradeoff between (1) increased parasitic power consumption resulting from the larger capacity 
11

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ratio caused by duct losses at design conditions; and (2) reduced duct losses during operation 

resulting from shorter runtimes in the case with the oversized system. Under these circumstances

the oversizing penalty increases in the presence of ducts, as indicated in Table 7.

Conclusions

Residential retrofits can be substantial investments that affect home energy use for years. 

Because many retrofits are expensive, homeowners often stage them over months or possibly 

years. Homeowners need to have access to the most accurate information so they can understand 

the energy impacts of retrofits in these situations.

This report describes a numerical case study of a typical 1960s vintage home in Houston, TX

that begins when the air conditioner is replaced. To analyze choices that homeowners or their 

contractors might consider, scenarios were simulated that also included duct and envelope 

improvements. These upgrades reduce the cooling load on the house and, if implemented at the 

same time the air conditioner is replaced, provide an opportunity for reducing the size of the air 

conditioner.

Comparison of simulation results for different scenarios demonstrated that the energy penalty

caused by an oversized air conditioner can be significant. However, the penalty is not due to unit 

cycling efficiency but rather to off-cycle parasitic power consumption. If the parasitic power is 

proportional to cooling capacity, the losses are substantial and support the conventional wisdom 

that having an oversized air conditioner results in an energy penalty.

Specific questions that are addressed are:

• How much of an air conditioner’s energy use is a result of cycling?

In many climates even a right-sized, single-speed air conditioner experiences significant

cycling because loads during many cooling season hours are low relative to loads at 

design conditions. Older air conditioners are sensitive to cycling and could have up to 

15% increased energy use. Newer air conditioners are less sensitive and experience only 

single-digit increases in energy use.

• What is the increase in energy use caused by additional cycling from an oversized air 

conditioner?

Oversizing does cause additional cycling, but this does not lead to significant increased

energy use because there are diminishing impacts after the seasonal cycling effect 

described above.

• Under what circumstances does oversizing lead to significantly increased energy use?

With off-cycle parasitic power consumption proportional to air conditioner capacity 

(from components such as controls and crankcase heaters) oversizing leads to significant

additional energy use, because oversized units are off more of the time.

• How are oversizing energy penalties influenced by the presence of ducts?

Without off-cycle parasitic power consumption, oversizing penalties are reduced in the 

presence of duct losses. With off-cycle parasitic power consumption proportional to air 
12

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

conditioner capacity, on the other hand, oversizing penalties increase when ducts are 

present.

Comparison of the simulated scenarios demonstrated that the oversizing penalty is significant if 

there are parasitic power losses. However, the penalty is not due to unit cycling efficiency as 

conventional wisdom might suggest. 
13

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

References

ASHRAE. (1989). Handbook of Fundamentals. Atlanta, GA: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers.

ASHRAE. (2004). Standard 152-2004. Atlanta, GA: American Society of Heating, Refrigerating 

and Air-Conditioning Engineers.

Cutler, D.; Winkler, J.; Kruis, N.; Christensen, C. (2013). Improved Modeling of Residential Air 

Conditioners and Heat Pumps for Energy Calculations. Golden, CO: National Renewable Energy 

Laboratory, NREL/TP-5500-56354.

DOE. (2002). “Right Size Heating and Cooling Factsheet.” Washington, D.C.: U.S. Department 

of Energy, DOE/GO-102002-1490.

DOE. (2013) EnergyPlus (version 8.0.0.008). www.energyplus.gov.

ECW. (2008). Central Air Conditioning Wisconsin: A Compilation of Recent Field Research. 

Madison, WI: Energy Center of Wisconsin, ECW Report Number 241-1.

Gorter, J. (2012). “HVAC Equipment Right-sizing: Occupant Comfort and Energy Savings 

Potential.” Energy Engineering 109: 59–75.

Henderson, H.; Huang, Y.; Parker, D. (1999). Residential Equipment Part Load Curves for use in 

DOE-2. Berkeley, CA: Lawrence Berkeley National Laboratory, LBNL-42175.

Henderson, H.; Parker, D.; Huang, Y. (2000). “Improving DOE-2’s RESYS routine: User 

Defined Functions to Provide More Accurate Part Load Energy Use and Humidity Predictions. 

Berkeley, CA: Lawrence Berkeley National Laboratory, LBNL-46304.

Henderson, H.; Raustad, R.; Rengarajan, K. (1991). Measuring Thermostat and Air Conditioner 

Performance in Florida Homes. Cocoa, FL: Florida Solar Energy Center, FSEC-RR-24-91.

Henderson, H.; Shirey, D.; Raustad, R. (2007). Closing the Gap: Getting Full Performance from 

Residential Central Air Conditioners, Task 4 - Develop New Climate-Sensitive Air Conditioner, 

Simulation Results and Cost Benefit Analysis. Florida Solar Energy Center, FSEC-CR-1716-07.

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. 

Washington, D.C.: U.S. Department of Energy, DOE/GO-102010-3141.

Mills, A.F. Heat and Mass Transfer. Irwin 1995, Chicago.

Proctor, J.; Katsnelson, Z.; Wilson, B. (1995). “Bigger is Not Better: Sizing Air Conditioners 

Properly.” Home Energy Magazine Online May/June.

Rhodes, J.; Stephense, B.; Webber, M. (2011). “Using energy audits to investigate the impacts of 

common air-conditioning design and installation issues on peak power demand and energy 

consumption in Austin, Texas.” Energy and Buildings 43(2011):3271–3278.

Sonne, J.; Parker, D.; Shirey, D. (2006). Measured Impacts of Proper Air Conditioning Sizing in 

Four Florida Case Study Homes. Cocoa, FL: Florida Solar Energy Center, FSEC-CR-1641-06.
14 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 

Appendix: Effects of Part Load Ratio 

The relatively small impact of oversizing on annual energy use from Figure 3 can be better 

understood by looking at the individual effects of CD and the part load fraction models described 

above. 

Figure 3 is a convenient graphical method for understanding the physical and mathematical 

trends associated with oversizing and cycling of equipment; it is not a substitute for using wholebuilding simulations to determine changes in annual cooling energy due to equipment sizing. The 

load graphs are histograms of the total cooling load at each PLR over a year using TMY3 

weather data for Houston. The air conditioner is a SEER 10 unit with no parasitic losses, 2 tons 

for the right-sized unit and 3.5 tons for the oversized unit, a capacity ratio of approximately 1.8. 

The part load fraction (PLF) is the fractional decrease in efficiency including cycling losses. A 

PLF of 1 means the unit operates at steady state efficiency; which occurs when the unit operates 

continuously (PLR = 1). A PLF of 0.9 means the unit operates at 90% of steady state efficiency, 

etc. The vertical lines are a graphical aid to help translate between the load histograms and the 

PLF curve over every PLR. The relative positions of the vertical lines are proportional to the 

capacity ratio. For example, with a 1.8x capacity ratio, if the vertical line corresponding to the 

right-sized system is at a PLR of 0.4, the vertical line corresponding to the oversized system will 

be at 0.22. This ratio is constant over every PLR. 

Figure 3. Histogram of loads for a right-sized unit and a 1.8x oversized unit. 

Dotted lines represent the PLR where the PLF is being calculated. 

This calculation occurs for every PLR. The effect on energy use can be weighted by the load at 

each PLR to gauge the annual oversizing impact e.g., the oversizing penalty at high loads is more 

important than the oversizing penalty at smaller loads. 

The difference in the value of PLF where it intersects the two vertical lines (the graph gives an 

example of PLR 0.36 for right sized and 0.21 for oversized, which correspond to the load-
15 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 

weighted average annual PLR for each capacity) is representative of the decrease in system 

efficiency from additional cycling from oversizing, a 3% decrease in this situation. This does not 

equate directly to a 3% increase in energy usage; however, it is indicative of the change in 

energy consumption due to oversizing. 

There can be significant cycling losses in both correctly sized and oversized systems because 

most hours of the year are spent at relatively low loads compared to design-condition loads; 

however, the impact of additional cycling from oversizing is more modest. 

Reduced Cycling Losses in Newer Air Conditioners 

Figure 4 is similar to Figure 3 except the load histograms are removed for clarity and another 

PLF curve is added which has a different cycling degradation coefficient. The cycling 

performance of equipment has improved significantly from 1990’s levels to today. SEER 10 

units sold in the 1990s had a CD ~0.2 whereas modern units can have a value closer to 0.07 

(Cutler et al. 2013). Figure 4 shows the reduction in PLF due to oversizing with a capacity ratio 

of 1.8 at a right-sized PLR = 0.36 for CD of 0.2 (poor cycling performance) is ~3% and for CD of 

0.07 (good cycling performance) it is ~1%. Cooling energy is not proportional to PLF but rather 

to the runtime fraction (RTF), however, PLF does represent a decrease in unit efficiency due to 

cycling. Therefore, Figure 4 provides a useful baseline when considering the effect of cycling 

performance on the energy penalty due to oversizing for both good and bad cycling performance. 

Figure 4. PLF for two different cycling degradation coefficients. 

The vertical lines are translated over every PLR. 

The ratio of the PLR for the two lines is the inverse of the capacity ratio (i.e. a 1.8x oversizing 

means the right-sized line has a PLR 1.8 times greater than the PLR of the oversized unit). The 

difference in the value of PLF where it intersects the two vertical lines represents the decrease in 

system efficiency from oversizing. A smaller cycling degradation coefficient, CD, reduces the 

impact of oversizing on PLF and hence on cooling energy used. 
16 

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications. 

Oversizing With and Without Off-Cycle Parasitic Power 

Figure 5 shows that the oversizing energy penalty depends heavily on whether there are parasitic 

power losses. The largest parasitic power draw on air conditioner units is a crankcase heater. 

These are standard equipment on heat pumps, which operate in colder conditions than air 

conditioner units; however, they may or may not be present on any particular air conditioner, 

depending on the age, manufacturer, capacity, performance, etc. A common method to control 

these heaters, if they exist, is to have the heater turn on if two conditions are satisfied: (1) the air 

conditioner is not operating; and (2) a reference temperature, typically the outdoor temperature at 

the condenser, is below a specified value. In the simulations where crankcase heaters were used 

in this study, the reference temperature was set arbitrarily high such that the heaters turned on 

whenever the air conditioner was off. This provides a sort of worst-case scenario in terms of 

maximizing the energy penalty associated with oversizing the unit. 

Figure 5. Comparison of PLF curves with (nonlinear) and without (linear) parasitic losses. 

The difference in PLF for each curve between the two vertical lines gives an idea on the oversizing 

penalty; this penalty should be evaluated over all PLR for which there is a load. For the nonlinear 

curve, there are substantial parasitic losses at PLR = 0 that do not affect PLF but do contribute to 

the annual oversizing penalty. For all PLR values, the parasitic loss curve (nonlinear) has a higher 

oversizing penalty. The effect is more pronounced at lower PLR values. 

Figure 5 shows two PLF curves, one with and one without parasitic losses included; both curves 

have CD = 0.2. The difference in PLF for each curve between the two vertical lines gives an idea 

of the oversizing penalty; this should be evaluated over all PLRs to get an approximate annual 

oversizing penalty. For all PLR values, the curve with parasitic losses (nonlinear) can be seen to 

have a steeper slope that will result in a higher oversizing penalty. The effect is more pronounced 

at lower PLR values. For the non-linear curve, there are substantial parasitic losses at PLR = 0 

that do not affect the PLF curve but do contribute to the annual oversizing penalty. These losses 

at PLR=0 occur when the air conditioner does not turn on during a simulation time step (1 hour 

for all simulations in this study). Because the air conditioner did not turn on, the PLR and hence 

PLF = 0, however, the parasitic losses still occurred during the entire hour and must be 

accounted for when calculating annual energy use and oversizing energy penalty. There are a 

large number of hours during the year for which PLR = 0, the parasitic losses during these times 

are the dominant factor in the oversizing penalty.